Method For Operating A Liquid-To-Air Heat Exchanger Device

ABSTRACT

A method for operating a liquid-to-air heat exchanger device comprises:
         determining a dew point temperature of ambient air;   determining whether the dew point temperature is higher than a temperature of the liquid, and as long as this is the case operating the heat exchanger device in a pulsed operation mode according to:   allowing the liquid to flow through a first heat exchanger stage during a predetermined period of time;   preventing that the liquid flows through the first heat exchanger stage, monitoring the temperature of the air at the exit of the first heat exchanger stage, said temperature indicating a first increase, remaining awhile constant and then showing a second increase;   detecting the second increase and terminating the preventing that liquid flows through the first heat exchanger stage once the second increase was detected.

TECHNICAL FIELD

The invention concerns a method for operating a liquid-to-air heatexchanger device.

BACKGROUND OF THE INVENTION

The method is suitable for operating a liquid-to-air heat exchangerdevice which comprises a passive heat exchanger stage in which the airis guided through a first flow channel extending in the verticaldirection and the liquid is guided through a second flow channel,wherein the two flow channels in this stage are separated by a thermallypassive separating wall. The term “thermally passive” means that theexchange of heat occurs without performing any work. The flow channelscontain a plurality of plate fins which are in good thermal connectionwith the thermally passive separating wall. The distances between theplate fins in the flow channel for the air are relatively small inrelation to the size of their surface so that the heat exchange isefficient.

If the air has a relatively high atmospheric humidity it may occurespecially on hot summer days that the dew point temperature of the airis higher than the temperature of the liquid. This leads to theconsequence that humidity contained in the air will deposit ascondensate on the plate fins. Since the overall size of the heatexchanger device is usually subject to narrow limits, it is difficult toform the plate fins in such a way that the produced water will drop anddrain off completely, especially in the case of vertical guidance of theair flow. This leads to the consequence that the water will increasinglyblock the intermediate spaces between the plate fins and willeffectively prevent further effective cooling of the air as a result ofthe thereof arising air resistance.

From GB 2461365, a central heating system with at least one radiator isknown, which can also be used for cooling. In cooling operation, heat iswithdrawn from the liquid circulating through the radiator by means of aheat exchanger. The extracted heat is supplied to a heat storage unit bymeans of a second heat exchanger. The two heat exchangers are part of acompressor-operated heat pump. In order to prevent that humidity cancondensate on the radiator, the dew point of the air is determined bymeasurement of temperature and humidity in the ambient environment ofthe radiator and when the determined dew point temperature moves towardsthe temperature of the radiator the cooling output is reduced.

From EP 508766, a method for controlling an air-conditioning system isknown, in which the temperature is determined in cooling operation inwhich condensation of water occurs on a test element and in which it isensured in this case that the temperature of the cooling liquid ishigher than the condensation temperature. This occurs for example bystopping the cooling operation.

The solutions known from this state of the art have the joint objectiveof preventing the accumulation of condensate and achieve this byreducing the cooling performance or interrupting cooling operation.

BRIEF DESCRIPTION OF THE INVENTION

The invention is based on the object of remedying the aforementionedproblem.

The object as mentioned above is achieved in accordance with theinvention by the features of claim 1. Advantageous embodiments areprovided in the dependent claims.

The invention relates to the operation of a liquid-to-air heat exchangerdevice, which comprises a first flow channel for the air and a secondfloor channel for the liquid. The heat exchanger device contains a firstpassive heat exchanger stage in which the first flow channel and thesecond flow channel are separated by a thermally passive separatingwall, and optionally a second heat exchanger stage in which the air isactively cooled or heated, i.e. by pumping of heat from one side to theother. The thermally passive separating wall consists of a material thatconducts heat very well. A matching condensate drainage system isadvantageously built into the second heat exchanger stage. The first andsecond flow channel can each also be a plurality of flow channelsextending in parallel. The flow channel or channels for the aircontain(s) plate fins.

The invention proposes a method in order to achieve the aforementionedobject. The method comprises two parts, namely a first part in which itis determined whether the dew point temperature of the air is higherthan the temperature of the liquid. This occurs by the following steps:

Determination of the dew point temperature of the ambient air, i.e. thedew point temperature of the air before it enters the first heatexchanger stage;

-   -   comparing the determined dew point temperature of the air with        the temperature of the liquid which is measured or transmitted        by a superordinate control device.

The dew point temperature of the air can be determined by the followingfor example:

-   -   Measurement of the temperature of the air and the humidity of        the air before the entrance of the air into the first heat        exchanger stage, and subsequently    -   determining the dew point temperature of the air from the        measured temperature and the measured humidity of the air.

The determination of the dew point temperature of the air from themeasured temperature T and the measured humidity of the air can occurfor example by means of a Mollier diagram. The dew point temperature,designated as T_(p1), can alternatively be determined by calculation bymeans of the equation

$T_{pl} = \frac{{241.2*{\ln \left( \frac{phi}{100} \right)}} + \frac{4222.03716*T}{241.2 + T}}{17.5043 - {\ln \left( \frac{phi}{100} \right)} - \frac{17.5043*T}{241.2 + T}}$

wherein the unit of measurement of the temperatures T and T_(p1) isdegrees Celsius and the air humidity phi is entered as a relative airhumidity stated in percent.

It is also possible to measure two other quantities of the h-x diagramof the air (h designates enthalpy, x designates absolute humidity), e.g.two from the dry bulb temperature, wet bulb temperature, specificenthalpy and density of the air, and therefrom the dew point temperatureof the air.

If and as long as the dew point temperature of the air is higher thanthe temperature of the liquid, the second part of the method isperformed, which consists of operating the heat exchanger device in anoperating mode designated as pulsed operation. The pulsed operationcomprises the following steps which are continuously repeated in thesame sequence:

-   -   allowing the liquid to flow through the first heat exchanger        stage during a predetermined period of time;    -   preventing that the liquid flows through the first heat        exchanger stage, and measuring and monitoring the air        temperature after exiting from the first heat exchanger stage,        wherein the air temperature measured after exiting from the        first heat exchanger stage indicates a first rise in the        temperature, and then remains for a certain period of time at a        level which is constant in good approximation and which        corresponds to the wet bulb temperature of the incoming air, and        then indicates a second rise in the temperature;    -   detecting the second temperature increase and terminating the        preventing that liquid flows through the first heat exchanger        stage once the second temperature increase was detected, and    -   repeating these steps as long as the dew point temperature of        the air is higher than the temperature of the liquid.

The condition whether the dew point temperature of the air is higherthan the temperature of the liquid is checked periodically oraperiodically in pulsed operation in that the first part of the methodis carried out.

In the pulsed operation, a phase of the removal of condensate byevaporation periodically follows an accumulation phase, while cooling ofthe air continues in an uninterrupted fashion. Although the pulsedoperation allows a temporary accumulation of water between the platefins, it still prevents blockage of the plate fins by condensate whichwould lead to blocking of the air flow, reduces the time of switchingoff the water flow to a minimum and thus increases the efficiency of theheat exchanger device.

In order to ensure that the method in accordance with the invention canbe performed, the heat exchanger device is equipped with the temperatureand humidity sensors necessary for this purpose.

If the heat exchanger device comprises a second active stage in whichheat is pumped between the liquid and the air by supply of energy, thestep of preventing that the liquid flows through the first heatexchanger stage further ensures according to a first variant that theliquid will also not flow through the second heat exchanger stage andthat the second heat exchanger stage is deactivated, or the step ofpreventing that the liquid flows through the first heat exchanger stageensures according to a second variant that the liquid is guided past thefirst heat exchanger stage (bypass), so that it can still flow throughthe second heat exchanger stage.

The invention is subsequently explained in more detail by means ofexemplary embodiments and the drawing. The drawings are not drawn toscale.

DESCRIPTION OF THE DRAWING FIGURES

FIGS. 1, 2 schematically show a side view or top view of the parts of aliquid-to-air heat exchanger device necessary for understanding theinvention, which heat exchanger device is set up for operation accordingto the method in accordance with the invention, and

FIG. 3 shows three diagrams for illustrating the method in accordancewith the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 schematically show a side and top view of the parts of aliquid-to-air heat exchanger device 1 necessary for the understanding ofthe invention, which heat exchanger device comprises a first passiveheat exchanger stage 2 and optionally a downstream active heat exchangerstage 3. The first heat exchanger stage 2 comprises at least one flowchannel 4 (preferably several thereof) for the air and at least one flowchannel 5 (preferably several thereof) for the liquid. The flow channels4 for the air and the flow channels 5 for the liquid are arranged inalternating succession and are separated by thermally passive separatingwalls which conduct heat well. The flow channels 4 for the air contain aplurality of plate fins 6 which are in good thermal connection with thethermally passive separating walls. The distances between the plate fins6 are small, so that heat exchange between the air and the liquid isefficient. The flow channels 4 for the air extend in the verticaldirection in this example.

The optional second active heat exchanger stage 3 can be formed indifferent ways. It can contain a cooling circuit with a compressor forexample, in which a cooling liquid circulates, wherein the air exchangesheat with the cooling circuit.

In the example shown in FIGS. 1 and 2, the second heat exchanger stage 3is formed in such a way that heat can be exchanged between the liquidand the air by supplying electrical energy, i.e. by means of at leastone Peltier element 10. The second heat exchanger stage 3 contains atleast one flow channel 7 for the air, at least one flow channel 8 forthe liquid, and the at least one interposed Peltier element 10 whichpumps heat from the liquid to the air when the air is to be heated andwhich pumps heat from the air to the liquid when the air is to becooled. The liquid is not subjected to any change in the aggregate statein this example. In the illustrated example, the air flows between platefins 9 which are arranged in parallel and which are in good thermalcontact with the at least one Peltier element 10.

The heat exchanger device 1 further comprises a valve 11 and optionallya bypass line 12 whose purpose will be described below in closer detail.

Persons skilled in the art often use the term “thermoelectric element”or the term “Peltier heat pump”as a synonym for the term “Peltierelement”. The thermoelectric elements are especially based on thePeltier effect, but they can also be based on another thermoelectriceffect such as the principle known as thermo tunnelling.

The heat exchanger device 1 comprises an inlet 13 and an outlet 14 whichcan be connected to an external liquid circuit. The liquid circulatingin the liquid circuit is heated or cooled by an external central deviceto a predetermined temperature. The liquid that is used is usually wateror a liquid on water basis, but it is possible to use any other suitableliquid. The flow channels 4 for the air extend in the verticaldirection. The flow channels for the liquid are designed as a linesystem which connects the inlet 13 and the outlet 14 to each other. Theheat exchanger device 1 further contains a fan and the necessary baffleplates and guide elements for the forced guidance of the air through thefirst heat exchanger stage 2 and the second heat exchanger stage 3 (ifpresent), and a drain 15 for the condensate accumulating in the secondheat exchanger stage 3. The direction of flow of the liquid is shown byarrows 16 and the direction of flow of the air is indicated by arrows17.

The heat exchanger device 1 further comprises the sensors which arenecessary for operation in accordance with the invention, which are atleast one temperature sensor 18 for the measurement of the temperatureand a humidity sensor 19 for measuring the humidity of the air, whichare arranged before the first heat exchanger stage 2, a temperaturesensor 24 measuring the temperature of the air which is arranged afterthe first heat exchanger stage 2, and a control device 21. Thetemperature of the liquid is either measured by means of a temperaturesensor 22 which is arranged at the inlet for example or is transferredfrom an external central device to the control device 21. The controldevice 21 evaluates the data transmitted by the sensors and controlsboth the flow rate of the liquid through the first heat exchanger stage2 and also the at least one Peltier element 10.

FIG. 3 shows three diagrams arranged one above the other, whichillustrate the following features of the method in accordance with theinvention in function of time t on the basis of an example.

The middle diagram shows the flow rate of the liquid through the firstheat exchanger stage 2. The flow rate of the liquid through the firstheat exchanger stage 2 is permitted for a predetermined period of timeT₁ and is then interrupted, wherein the interruption in the flow of theliquid through the first heat exchanger stage 2 occurs either by closingthe valve 11 or, if there is a bypass line 12, by changing over thevalve 11, so that the liquid flows through the bypass line 12 and istherefore guided past the first heat exchanger stage 2.

The bottom diagram shows the current flowing through the at least onePeltier element 10 in the case that the interruption of the flow of theliquid through the first heat exchanger stage also produces theinterruption of the flow of the liquid through the second heat exchangerstage 3. The current flowing through the at least one Peltier element 10is deactivated either simultaneously or with a time delay whenever theflow of the liquid through the first heat exchanger stage 2 isinterrupted, so that the at least one Peltier element 10 will notoverheat. In the other case that the flow of the liquid through thesecond heat exchanger stage 3 is not interrupted, the at least onePeltier element 10 is not deactivated.

The upper diagram shows the progression of the temperature of the airafter exiting the first heat exchanger stage 2, i.e. the progression ofthe temperature measured by the temperature sensor 20. The illustrationclearly shows a first temperature increase 23 (in the example from 18°C. to approximately 22° C.), an approximately constant level 24 and asecond temperature increase 25 (in the example from approximately 22° C.to approximately 27° C.).

The progression of the temperature as shown in the diagram consists ofthe following repeated phases A-D:

Phase A: The flow of the liquid through the first heat exchanger stage 2is not interrupted. The air is cooled, in the example to approximately18° C. Water gradually condensates between the plate fins 6, whichincreasingly increases the flow resistance of the air.

Phasen B to D: The flow of the liquid through the first heat exchangerstage 2 is interrupted.

Phase B: The temperature of the air increases to the approximatelyconstant level 24.

Phase C: The temperature of the air remains at the level 24, since thewater accumulated between the plate fins 6 evaporates and cools the airadiabatically in this process.

Phase D: The temperature of the air increases further once the water hasevaporated between the plate fins 6.

FIG. 3 shows the pulsed operation very clearly. Since the duration ofthe individual cycles (a cycle comprises a sequence of the phases A-D)typically lies in the range of a few minutes or a few 10 minutes and thedew point temperature of the air usually only changes slowly, the dewpoint temperature only needs to be measured occasionally during thepulsed operation, e.g. once per half hour or hour, or also in otherintervals.

1. A method for operating a liquid-to-air heat exchanger device, inwhich air flows through at least one first flow channel at least in afirst passive heat exchanger stage (2), which flow channel comprisesplate fins, and a liquid flows through at least one second flow channelwhich is separated from the at least one first flow channel by athermally passive separating wall, comprising the following steps:determining a dew point temperature of ambient air; determining whetherthe dew point temperature of the ambient air is higher than atemperature of the liquid, and if this is the case operating the heatexchanger device in an operating mode designated as pulsed operationaccording to the following steps: allowing the liquid to flow throughthe first heat exchanger stage during a predetermined period of time;preventing that the liquid flows through the first heat exchanger stage,and measuring and monitoring the temperature of the air after exiting ofthe air from the first heat exchanger stage, wherein the temperature ofthe air measured after exiting the first heat exchanger stage indicatesa first temperature increase, remains subsequently at an approximatelyconstant level for a specific period of time, and then shows a secondtemperature increase; detecting the second temperature increase andterminating the preventing that liquid flows through the first heatexchanger stage once the second temperature increase was detected, andrepeating these steps as long as the dew point temperature of theambient air is higher than the temperature of the liquid.
 2. The methodof claim 1, wherein the dew point temperature of the ambient air isdetermined by measuring the temperature of the air and the humidity ofthe air before entrance of the air into the first heat exchanger stage,and determining the dew point temperature of the air from the measuredtemperature and the measured humidity of the air.
 3. The method of claim1, in which heat is pumped between the liquid and the air by supply ofenergy in a second active heat exchanger stage, wherein the step ofpreventing that the liquid flows through the first heat exchanger stagealso ensures that the liquid does not flow through the second heatexchanger stage, and that the second heat exchanger stage isdeactivated.
 4. The method of claim 1, in which heat is exchangedbetween the liquid and the air by supply of energy in a second activeheat exchanger stage, wherein the step of preventing that the liquidflows through the first heat exchanger stage ensures that the liquidbypasses the first heat exchanger stage so that it still flows throughthe second heat exchanger stage.
 5. The method of claim 2, in which heatis pumped between the liquid and the air by supply of energy in a secondactive heat exchanger stage, wherein the step of preventing that theliquid flows through the first heat exchanger stage also ensures thatthe liquid does not flow through the second heat exchanger stage, andthat the second heat exchanger stage is deactivated.
 6. The method ofclaim 2, in which heat is exchanged between the liquid and the air bysupply of energy in a second active heat exchanger stage, wherein thestep of preventing that the liquid flows through the first heatexchanger stage ensures that the liquid bypasses the first heatexchanger stage so that it still flows through the second heat exchangerstage.